Screws have long been used to secure objects to one another. Depending on the requirements of a particular application, the screw may take on many different shapes and sizes. Although screws have been produced in enormous variety, the vast majority create axial compression along the length of the screw by virtue of drawing the lower surface of a head down against the surface of one object using threads embedded in an adjacent object, as, for instance, occurs in a wood screw.
Although traditionally configured screws are suitable for most purposes, in some instances the presence of the head is unacceptable for functional, structural or cosmetic reasons. Various types of headless screws have been developed to address some of the applications where a traditional screw is less than optimal. One early example of such a screw is shown in U.S. Pat. No. 187,023 to Russell which employs a variable pitch thread to generate compression without relying on a head. Although the screw of Russell will generate some compression, its effectiveness is limited by the fact that the threads disposed toward the trailing end achieve little purchase because of the stripping that occurs due to the preceding threads of different pitch.
U.S. Pat. No. 4,175,555 to Herbert overcomes Russell's problem of reaming of female threads by subsequent threads on the screw. In particular, the Herbert screw has a region of large pitch and small diameter thread near the leading end and a region of smaller pitch and larger diameter thread near the trailing end, with the regions being separated by an unthreaded section. Unfortunately, the Herbert screw suffers from a number of other disadvantages. In the Herbert screw, the leading threads have a smaller diameter than the trailing threads. This is necessary to permit the leading threads to pass through the relatively large bore in the near bone fragment and engage the smaller bore in the remote bone fragment. The larger trailing threads then engage the larger bore in the near bone fragment As a result of this arrangement, any stripping of the threads cut into the bones during installation of the screw occurs in the remote bone. If the stripping occurred in the bore in the near bone fragment, a screw having a head thereon could still be used to compress the fracture even though the near bore was stripped; however, when stripping occurs in the bore in the remote bone, a standard screw with the head thereon cannot be used and another bore must be drilled.
Further, the Herbert screw must be correctly positioned, i.e., it is imperative that the fracture intersect the unthreaded central portion of the Herbert bone screw when the same is installed. Thus, the Herbert screw is not suitable for treating fractures that are very near the surface of the bone where the hole is to be drilled In addition, because the Herbert screw is not threaded entirely along the length thereof the purchase obtained by the screw in the bone is not as good as with a screw threaded along the entire length. Also, two bores of different sizes must be drilled to install the Herbert screw rather than a single bore.
Yet another problem with the Herbert screw is the stripping that can occur if additional tightening occurs after the screw has drawn the bone fragments together. While the bone fragments are being drawn together, trailing threads K.sub.6 all follow a single path through the near fragment Similarly, leading threads J.sub.6 all follow a single path through the remote fragment When, however, the bone fragments make contact, the two sets of threads can no longer move independently. Further rotation of the Herbert screw after contact between the fragments can cause the leading threads to strip out as they attempt to move forward through the distal bone fragment faster than the trailing threads will allow. See The Herbert Bone Screw and Its Applications in Foot Surgery. The Journal of Foot and Ankle Surgery No. 33, Vol. 4., 1994, pages 346-354, which reports on a study that found compression of 10 kg after only two complete turns of the trailing threads engaged in the near bone fragment Each subsequent revolution lead to a decrease in compressive force. Thus, care must be taken not to over-tighten the Herbert screw.
The various problems of the Herbert screw are addressed by the screw shown in U.S. Pat. No. 5,871,486, issued Feb. 16, 1999, to Huebner, which is herein incorporated by reference. The Huebner screw, sold under the trademark ACUTRAK, in most versions, is fully threaded and has a changing pitch over the entire length. The outside diameter of the thread tapers from front to rear so that as the trailing threads ream the tracks left by the leading threads due to the pitch change, the trailing threads are expanding outward into undisturbed material. The ACUTRAK screw can be driven in as far as desired without reduction in compression because of the expanding thread diameter along its length. In addition, the screw generates compression over the entire length, rather than only at the tip and tail as with Herbert. Thus, the ACUTRAK screw can be used to repair fractures anywhere along its length.
Although the ACUTRAK screw was a major improvement over the Herbert screw, installation of the ACUTRAK screw requires careful attention. In particular, the screw is typically installed in a tapered hole that has been pre-drilled. Drilling this tapered hole can be difficult because the tapered drill bit sometimes clogs with bone and tapered bits require more pressure to feed than a similar straight drill. Moreover, for best results, the depth of the hole should generally match the length of the screw, requiring the surgeon to drill the hole accurately. The other area of concern during installation is the torque needed to drive the screw. When the screw reaches the point where the root and hole taper match, the driving force increases substantially. Similarly, in dense bone, the driving force may make installation difficult.